1. Field of the Invention
The present invention relates to an acoustic wave filter device having balanced-unbalanced conversion functions, and particularly relates to an acoustic wave filter device in which a single-port acoustic wave resonator is connected to a longitudinally-coupled resonator type acoustic wave filter unit.
2. Description of the Related Art
In general, a band filter including a balanced-unbalanced conversion function is required for an RF stage of a cellular phone, for example. As such a band filter, a surface acoustic wave filter device having a balanced-unbalanced conversion function is generally used.
In Japanese Unexamined Patent Application Publication No. 2001-308672 below, for example, a surface acoustic wave filter device having an electrode configuration as shown in FIG. 13 is disclosed. In a surface acoustic wave filter device 1001, an electrode configuration shown in FIG. 13 is connected between an unbalanced terminal 1002 and first and second balanced terminals 1003 and 1004. A single-port surface acoustic wave resonator 1005 is connected to the unbalanced terminal 1002. A second terminal of the surface acoustic wave resonator 1005 is connected to first and second longitudinally-coupled resonator type surface acoustic wave filters 1006 and 1007. Both of the surface acoustic filters 1006 and 1007 are 3-IDT longitudinally-coupled resonator type surface acoustic wave filters.
IDT electrodes located at the middle of the surface acoustic wave filters 1006 and 1007 are connected to each other and further connected to the surface acoustic wave resonator 1005. On the other hand, IDT electrodes arranged so that the middle IDT electrode of the surface acoustic filter 1006 is sandwiched between the IDT electrodes are connected to each other and further connected to the first balanced terminal 1003 through a single-port surface acoustic wave resonator 1008. Similarly, IDT electrodes arranged so that the middle IDT electrode of the surface acoustic filter 1007 is sandwiched between the IDT electrodes are connected to each other and further connected to the second balanced terminal 1004 through a SAW resonator 1009. Note that the surface acoustic wave filters 1006 and 1007 are configured such that a phase of a signal transmitted through a first signal line 1010 which connects the surface acoustic filter 1006 to the first balanced terminal 1003 is shifted by 180 degrees relative to a phase of a signal transmitted through a second signal line 1011 which connects the surface acoustic filter 1007 to the second balanced terminal 1004.
Furthermore, a surface acoustic wave resonator 1012 is connected between the first and second signal lines 1010 and 1011. Specifically, the single-port surface acoustic wave resonator 1012 has a first bus bar connected to the first signal line 1010 and a second bus bar connected to the second signal line 1011.
A resonant frequency of the single-port surface acoustic wave resonator 1012 is set in a lower side of pass bands of the surface acoustic wave filters 1006 and 1007, and accordingly, an out-of-band attenuation amount is increased. Furthermore, according to Japanese Unexamined Patent Application Publication No. 2001-308672, the surface acoustic wave resonator 1012 is used to connect the first and second signal lines 1010 and 1011 to each other, the same influence is applied to the balanced terminals 1003 and 1004. Accordingly, deterioration of balancing can be suppressed.
On the other hand, according to Japanese Unexamined Patent Application Publication No. 2004-96244, a reactance component is added to one of first and second balanced terminals included in a surface acoustic wave filter device having a balanced-unbalanced conversion function. By adding the reactance component, phase balancing is improved. In FIG. 19 of Japanese Unexamined Patent Application Publication No. 2004-96244, comb-shaped electrodes are employed in a configuration to add the reactance component. That is, capacitance of the comb-shaped electrodes makes it possible to add the reactance component to one of the balanced terminals.
According to the surface acoustic wave filter device 1001 of Japanese Unexamined Patent Application Publication No. 2001-308672, since the surface acoustic wave resonator 1012 is connected, the out-of-band attenuation amount is increased. However, among the first and second bus bars of the surface acoustic wave resonator 1012 which are located at opposite sides relative to the surface acoustic wave resonator 1012, the first bus bar is connected to the first signal line 1010 and the second bus bar is connected to the second signal line 1011. That is, since lines extend from the first and second bus bars, a wiring configuration on a piezoelectric substrate is likely to be complicated. Therefore, it is difficult to enhance miniaturization. Furthermore, as described above, since substantially the same influence of the connection of the surface acoustic wave resonator 1012 is applied to the first and second balanced terminals 1003 and 1004, balancing is not considerably deteriorated. However, this configuration does not positively improve the balancing. Accordingly, in the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2001-308672, balancing between signals transmitted to the first and second balanced terminals 1003 and 1004 is not improved.
On the other hand, in the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2004-96244, the reactance component is added to one of the balanced terminals since the configuration including the comb-shaped electrodes is employed. Therefore, phase balancing is improved. However, although the balancing is improved, there arises a problem in that insertion loss is increased due to an increase of parasitic capacitance caused by the addition of the reactance component. Furthermore, since the reactance component is added, an area of electrodes in a piezoelectric substrate is increased, and therefore, a surface acoustic wave filter chip becomes large.